Quick and dirty method:
wire the alternator output to the battery side of the cutoff switch. no need to fiddle with field wires, ECUs, fuel pump relays, nothing.

when you flip the switch, there is ZERO power anywhere under the hood. Zip. Zilch. Nada. No fuel pump, no ignition, no ECU. Absolutely foolproof and there's no freewheeling alternator feeding back into the circuit to fry the ECU.

Even if the alternator is self-fielding, all it's doing is charging the battery that is already disconnected. Took me about 6' of #8 wire and 30 min to accomplish the job.